FIG. 9(a) is a cross-sectional view, showing a schematic configuration of a hetero-junction field effect transistor (hereinafter referred to as HJFET) according to a conventional technology, which is typically described in Patent Document 1. A gallium nitride (GaN) channel layer 102 and an aluminum gallium nitride (AlGaN) electron-supplying layer 103 are consecutively formed on a substrate 100. The electron-supplying layer 103 creates no lattice match with the GaN channel layer 102, and thus a growth thereof to a sufficient thickness for avoiding an influence of the surface cannot be achieved. Therefore, an n-type indium aluminum gallium nitride (InAlGaN) cap layer 104, which is capable of creating lattice match with the GaN channel layer 102, is employed to coat over an upper surface of the electron-supplying layer 103.
A gate electrode 105 is formed in a concave portion provided in the cap layer 104 so as to be in contact with the electron-supplying layer 103. A source electrode 106 and a drain electrode 107 are formed on the cap layer 104. In this structure, InAlN ratio in the n-type InAlGaN cap layer 104 is selected to be 1.5 times or more of the AlN ratio in the electron-supplying layer 103, so that polarized charge in the cap layer 104 is not lower than polarized charge in the electron-supplying layer 103. This results in reduced potential barrier between the cap layer 104 and the electron-supplying layer 103, achieving a reduction of the ohmic resistance.
FIG. 9(b) is a diagram of energy band, showing a conduction band energy distribution corresponding to the ohmic contact of FIG. 9(a). Ordinate represents energy and abscissa represents depth. Abscissa in the left side represents the ohmic electrode, and the right side represents the channel layer. A depleted layer extends over an interface of the semiconductor that is in contact with the electrode, creating a potential barrier for electron.
In addition to above, prior art literatures related to the present invention also includes Non-Patent Document 1, in addition to Patent Document 1.
[Patent Document 1]
    Japanese Patent Laid-Open No. 2002-289,837[Non-Patent Document 1]    Ambacher et al., J. Phys.: Condense Matter, Vol. 14, Page 3399.